In recent years, for protection of natural environment, environmental pollution such as photochemical smog or global warming due to discharge of organic solvents, has been taken up as a problem, and discharge regulation or emission control is being implemented. Particularly in Europe, such control is advanced, and also in Japan, Air Pollution Control Law was revised in 2006, and legal discharge regulations were implemented.
In order to meet such legal regulations, also in the field of coating materials, efforts have been made to develop a coating method to increase the coating efficiency to effectively utilize a coating material or to introduce a high solid type coating material by increasing the solid content in a coating material thereby to reduce a solvent to be discharged. However, with such a high solid type coating material, there is a limit in reduction of the solvent.
Heretofore, compositions for aqueous coating materials have been known wherein various synthetic resins of e.g. alkyd type, acryl type, polyester type, polyvinyl acetate type or epoxy type, are dispersed or dissolved in water. An aqueous coating composition employing a crosslinkable fluorinated copolymer having hydroxyl groups, as such a synthetic resin or as a part of such a synthetic resin, is widely known, since it is excellent in weather resistance.
As a fluorinated copolymer to be used for a composition for aqueous coating material, one having some of hydroxyl groups esterified to introduce carboxyl groups in an organic solvent and further having some or all of such carboxyl groups neutralized by a basic compound, in order to increase the affinity to water, is known (Patent Document 1).
Further, a fluorinated copolymer obtained by emulsion polymerization is also known (Patent Document 2). However, in the aqueous dispersion or aqueous solution of the fluorinated copolymer in Patent Document 1, a large amount of an organic solvent used in the step of introducing carboxyl groups will remain. Further, if the remaining organic solvent is removed, the storage stability tends to deteriorate, and sedimentation tends to take place as the time passes.
On the other hand, in a case where a fluorinated copolymer obtained by emulsion polymerization is used as in Patent Document 2, an emulsion coating material having a low solvent concentration can be obtained. However, the emulsion coating material has had a problem that the film-forming property is poor, or due to the remaining emulsifier, the water resistance or adhesion tends to be poor.
Patent Document 1: JP-B-1-29488
Patent Document 2: Japanese Patent No. 3,356,803